Saturday, February 2, 2013

The Curious Case of Carnivorous Plants

Neha Patel

Mario jumping to his shrinkage.

For many of us, the introduction of carnivorous plants started with having our hero, Mario, be seemingly chewed up by the laughing plants that made an appearance from the green tubes at the worst moment (almost got that new high score!). For others, it may have been watching the extraterrestrial plant, Audrey II, in the musical Little Shop of Horrors steadily eat more people to eventually take over the world.

In reality, the species of carnivorous plants discovered
thus far feed mostly on insects (no man-eaters…for now), and have specialized structures
and enzymes to attract, trap, and digest their prey. These plants grow in
environments that have nutrient-poor soils, and so evolved their carnivorous
abilities in order to obtain essential nutrients such as nitrogen and
phosphorous. The Venus flytraps (genus Dionaea) are some of the most recognizable of the
carnivorous plants, and can even be bought to spruce up your household!

Venus Flytrap

Another type of carnivorous plant is the pitcher plant.
Of particular interest to this blog post is the genus Nepenthes, which has been previously studied in terms of
structure, and is more recently being researched for the enzymatic
components necessary for prey digestion. The structure of the “pitcher” is a
modified leaf, which is essentially rolled up and fused together. Insects are
lured to this trap by colors and patterns, the pollen, and/or the sweet smell
of the fluid inside the plant. The upper rim of the “pitcher,” called the
peristome, has an ideal surface topography that causes insects to fall into the
structure when they land to investigate. The inside walls of the structure are waxy, which does not allow the prey to climb back out. After the prey has been
suckered into the fluid at the base, it stimulates the release of digestive enzymes into the fluid to
effectively break down and absorb the organism.

Nepenthes spectabilis x ventricosa

The existence of enzymes in carnivorous plants has been known since the time of Darwin, and later studies were able to detect the presence of a variety of enzymes that would be important in breaking down cells, such as proteases, nucleases, and phosphatases. This is well and dandy, but it is important to note that insects have an exoskeleton made of a compound called chitin. If the plant is not able to break that down, then it won’t be able to get to the good stuff. So, Eilenberg et al. set out to find the enzymes that would break down chitin—or chitinases. The lab isolated the proteins from the pitcher fluid, and evaluated expression of the mRNA of these proteins before and after a chitin injection. In 2006, the lab published their findings that the plant had many different chitinases that were expressed, and that these enzymes were specific to breaking down certain variations of this molecule.

Chitin molecules can be chains of different lengths.

The pitcher plant is ideal to work with to study carnivorous plants because the environment inside the pitcher is sterile. This means that whatever proteins that are being secreted are only from that plant, and not from something else, such as a microorganism. The maintenance of the sterility is hinted at in the Eilenberg et al. paper. One of the chitinases they found is always expressed, and it is what they referred to as a “housekeeping chitinase.” In 2008, Hatano and Hamada published a paper on the analysis of the total expressed proteins, or proteome, in the pitcher fluid of a different Nepenthes species. They found the presence of a thaumatin-like protein, which is a pathogenesis-related protein that is expressed in plants when they are infected by pathogens. This protein also happens to be a chitinase. Now is probably a good time to mention that fungi have cell walls that are made of chitin! This suggests that there are connections between the sterility of the inside of the pitcher structure, and the presence of chitinases.

So why the heck should we care about these enzymes in carnivorous plants, especially when the bulk of them are used to liquefy their meals? As our previous researchers discovered accidentally (ahh, science), some of the chitinases may have antimicrobial properties, specifically against fungi. This has a possibility of being useful in plant biotechnology for the purpose of making plants more resilient to pathogens, if you are into genetically modified organisms (agribusinesses are). Maybe Mario knew what he was doing when he jumped into the plant—the prospect of fortune via agricultural pursuits was too hard to resist!

Hi Gennarina, thanks for the comment! I read in one of the papers there have been studies done on secondary metabolites found in the pitcher fluid that have been shown in a bioassay to be anti-fungal against some human pathogens. It would be awesome if these plants were able to be used for the development of HIV/AIDS treatments.

This is very interesting! I'm wondering if any of the literature has discussed any possible environmental effects if the antifungal technology was perfected and used for agribusiness? Could certain fungi become resistant to some of the genetically (or chemically) modified crops?

Very interesting! What about any antibacterial compounds? Since it smells sweet and therefore probably has sugar and no fungi to compete with, it seems like this would be an ideal environment for bacteria to grow.